The area lies between the Kerio and Suguta Rivers in southern Turkana. Metamorphic basement rocks are exposed along the axis of an anticlinal arch which is overlain by a succession of volcanics and associated sediments, up to 2,000 ft. (610 m.) in thickness west of it, and up to 11,000 ft. (3,400 m.) east of it. A basal grit formation passes up into basalts dating from 16.6 m.y., which are then overlain uncomfortably by a thick succession of phonolites. The latter constitute the Tiati Hills east of the basement inlier. The phonolites are succeeded by basalts which are in turn overlain by a group of large trachyte central volcanoes, consisting of trachyte lavas and subordinate pyroclastics, including welded tuffs, erupted from multicentre source zones. Minor basalt eruptions are the last manifestation of volcanic activity older than the shield volcanoes of the axial zone. The latter are only represented in the present area by the distal portions of airfall tuff showers erupted from them. Structurally, the older eruptions are related to a monocline active at first to the west of the present watershed, but migrating steadily eastwards with time. In this manner, east of the basement arch, the volcanics become younger towards the centre of the developing rift valley. The younger volcanics are cut by normal faults trending generally north-northeastwards. A wide range of rock types is present, including basanitoids, basalts, mugearites, phonolites and trachytes among the lavas, and pumice tuffs and welded tuffs among the pyroclastics. The basic and salic lavas are present in approximately equal volume, and together are far more abundant than intermediate lavas. Their alkalinity decreases generally with time. The distribution of each major stratigraphic unit is related to inferred cycles of magmatism.